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Finite element analysis of long posterior transpedicular instrumentation for cervicothoracic fractures related to ankylosing spondylitis
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Surgical Sciences, Orthopaedics.ORCID iD: 0000-0002-2724-6372
KTH Stockholm.
KTH Stockholm.
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Surgical Sciences, Orthopaedics.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Background: Spinal fractures related to AS are often treated by long posterior stabilisation. The biomechanical rationale behind is the neutralisation of long lever arms in the ankylosed spine to avoid non-union or neurological deterioration. Despite the widespread application of long posterior instrumentation it has never been investigated in a biomechanical model. The objective of this study is to develop a finite element model for spinal fractures related to AS and to establish a biomechanical foundation for long posterior stabilisation of cervicothoracic fractures related to ankylosing spondylitis (AS).

Methods: An existing finite element-model (consisting of two separately developed models) including the cervical and thoracic spine were adapted to the conditions of AS (all discs fused, C0-C1 and C1-C2 mobile) and a fracture at the level C6-C7 was simulated. Besides a normal spine (no AS, no fracture) and the uninstrumented fractured spine four different posterior transpedicular instrumentations were tested: 1. Fracture uninstrumented, 2. Short instrumentation C6-C7, 3. Medium instrumentation C5-T1, 4. Long instrumentation C3-T3, 5. Skipped level long instrumentation C3-C6-C7-T3.

Three loads (1.5g, 3.0g, 4.5g) were applied according to a specific load curve. Kinematic data such as the gap distance in the fracture site were obtained. Furthermore the stresses in the ossified parts of the discs were evaluated.

Findings: All posterior stabilisation methods could normalise the axial stability at the fracture site as measured with gap distance. With larger accelerations than 1.5g ,  it was seen that the longer instrumentations resulted in lesser maximal gap distance than the Short instrumentation. The maximum stress at the cranial instrumentation end (C3-C4) was slightly greater if every level was instrumented, than in the skipped level model. The skipped level instrumentation achieved similar rotatory stability as the long multilevel instrumentation.

Interpretation: The FE model developed simulated a spinal fracture at C6-C7 level. Skipping instrumentation levels without giving up instrumentation length also reduces the stresses in the ossified tissue within the range of the instrumentation and does not decrease the stability in a finite element model of a cervicothoracic fracture related to AS. Considering the risks associated with every additional screw placed, the skipped level instrumentation has advantages with regard to patient safety. The effects of the degree of osteoporosis, screw placement and pre-existing kyphosis on the construct stability were not investigated in this study and should be a matter of further research. 

National Category
Orthopedics Bio Materials
Research subject
Orthopaedics
Identifiers
URN: urn:nbn:se:uu:diva-307366OAI: oai:DiVA.org:uu-307366DiVA: diva2:1046378
Available from: 2016-11-14 Created: 2016-11-14 Last updated: 2016-11-14
In thesis
1. Spinal fractures related to ankylosing spondylitis: Epidemiology, clinical outcome and biomechanics
Open this publication in new window or tab >>Spinal fractures related to ankylosing spondylitis: Epidemiology, clinical outcome and biomechanics
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Background: Spinal fractures related to ankylosing spondylitis (AS) are often associated with serious complications. Therefore, knowledge of the incidence, best treatment, outcome, and prevention would assist in improving current guidelines.

Objectives: This thesis aims at (1) analysing the complications and mortality of surgical treatment, (2) mapping the incidence and treatment modalities for these patients in Sweden, as well as (3) investigating the putative preventive effect of biological disease modifying anti-rheumatic drug (bDMARD) therapy on spinal fractures related to AS.

Methods: Merged multiple national registries were used to identify predictors of mortality and spinal fractures in patients with AS. Beyond that a finite element model (FEM) was designed to simulating a cervicothoracic fracture related to AS.

Results and Conclusions: During the last two decades an increase of the incidence of vertebral fractures in patients with AS was observed. With the introduction of bDMARD treatment of AS was revolutionised and quality of life and function improved.  It seems that the improved quality of life and function in these patients does not correlate with a reduced fracture risk. Still, for the first time a beneficial effect of bDMARD with regard to spinal fracture occurrence was provided. The risk of spinal fractures was not reduced, but the debut of a spinal fracture was delayed with bDMARD. Since for this study the observation interval was only a decade, a future follow-up should revisit the effect of bDMARD on spinal fractures related to AS.

Furthermore, it was shown that posterior stabilisation is an effective method for restoring stability without the necessity of additional external fixation. Most likely the early rehabilitation reduced pulmonary complications, which in turn reduced early mortality of these fractures. The FEM could be used to identify the most appropriate implant configuration, since no well-established cadaver models exist.

Clinical Trial Registration: ClinicalTrials.gov, Identifier NCT02840695.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. 80 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine, ISSN 1651-6206 ; 1277
Keyword
ankylosing spondylitis, spinal fractures
National Category
Orthopedics
Research subject
Orthopaedics
Identifiers
urn:nbn:se:uu:diva-307373 (URN)978-91-554-9751-4 (ISBN)
Public defence
2017-01-20, Gullstrandsalen, Akademiska sjukhuset, Ing 70, Uppsala, 09:00 (English)
Opponent
Supervisors
Available from: 2016-12-09 Created: 2016-11-14 Last updated: 2016-12-28

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